This invention relates to a process and apparatus for producing portland and other hydraulic cements, and more particularly to a process and apparatus for the making of such cements utilizing electrical energy.
The hydraulic cements have long been recognized as an important group of cementing materials which are used principally in the construction industry. These cements have the special property of setting and hardening under water. The essential components of the cements are lime (CaO), silica (SiO.sub.2), alumina (Al.sub.2 O.sub.3), and the compounds derived therefrom. In the presence of water, these compounds react to form, ultimately, a hardened product containing hydrated calcium and alumina silicates. The hydraulic cements include portland cement as well as high alumina cement, hydraulic lime, and other lesser known cements.
Of all the hydraulic cements, portland cement is by far the most important. Portland cement is a major construction material that is utilized in practically all concrete as well as in most of the masonry mortars. The principal components of portland cement are tricalcium silicate (3CaO.SiO.sub.2), dicalcium silicate (2CaO.SiO.sub.2), and tricalcium aluminate (3CaO.Al.sub.2 O.sub.3), all of which, when in a ground or powdered condition, will react with water to form a hard, stone-like substance held together with intermeshed crystals. Other compounds, such as magnesium oxide (MgO) and tetracalcium aluminoferrite (4CaO.Al.sub.2 O.sub.3.Fe.sub.2 O.sub.3), which are present in portland cement, do not exhibit any cementitious properties. The exact composition of portland cement is defined in A.S.T.M. Standard Specifications which are accepted by the industry.
Portland cement in most plants producing the cement is obtained by finely intergrinding lime and silica containing materials and heating the mixture within a rotary kiln to the point of fusion. Fusion occurs at or about 1290.degree. C., the precise temperature depending upon the chemical composition of the feed materials and the type and amount of fluxes that are present in the mixture. The principal fluxes are alumina (Al.sub.2 O.sub.3) and iron oxide (Fe.sub.2 O.sub.3), and these fluxes enable the chemical reactions to occur at relatively lower temperatures. Normally the lime is obtained from natural calcareous deposits such as limestone, marl, and aragonite. Under certain conditions, lime may be derived from industrial by-products such as phospho-gypsum, a pulverulent calcium sulfate which may be obtained from the manufacture of phosphoric acid. The silica and fluxes, on the other hand, are normally derived from natural argillaceous deposits such as clay, shale, and sand.
More specifically, to manufacture portland cement, an argillaceous material and a calcareous material are crushed, mixed, and interground to a fine power, with the proportions of the two materials and the composition of each being maintained within narrow limits. The mixture passes into the upper end of a rotary kiln where it is heated eventually to the fusion point. However, before this point, water and carbon dioxide are driven off. As the hottest region is approached, a part of the interground mixture of materials melts and chemical reactions take place between the constituents of the raw mixture. In the course of these reactions new compounds are formed. After passing the hottest region, the compounds fuse and form a clinker. The clinker then is discharged into some form of a cooler. When cool, the clinker is mixed with a carefully controlled quantity of gypsum, and the mixture is ground to a very fine powder. That finely ground powder is the portland cement of commerce.
Rotary kilns vary in length and diameter. They revolve slowly (one turn in every one to two minutes or more) and, as they are slightly inclined, the charge slowly travels downwardly toward the hot end of the kiln. Being heated from its lower end, a rotary kiln develops its hottest temperatures in a rather narrow zone of the kiln, with the temperature becoming progressively lower toward the upper end. At no time does the entire mixture in the rotary kiln, even in the hottest zone, become molten, Special refractories are required, especially for the hot zone at the lower end, and once the kiln is fired it must remain in operation continuously, otherwise the expensive refractory will be damaged by thermal shocks upon cooling and reheating. Attempting to operate a rotary kiln above its normal operating temperature range will result in a high percentage of the feed mixture becoming liquid at one time and running uncontrollably out of the kiln. It will also cause severe damage to the refractories, to the kiln shell, and to the clinker cooler.
Generally, a rotary kiln is heated by burning a fossil fuel at its lower end, with the hot combustion gases traveling up the kiln. Heat energy is transferred to the downwardly moving raw feed by direct contact and indirectly by heating the refractory lining. As the raw materials become dried, heated, and partly calcined by the hot gases, some of the finer particles are picked up and transported out of the kiln as kiln dust.
The kiln dust usually contains some alkalies, primarily in the form of compounds of sodium and potassium, for they are usually found in the raw feed and also in coal which is used as a fuel. Also the raw feed and fuel often contain sulfur which volatilizes and enters the gas stream where it usually combines with lime and alkalies to form sulfates. The kiln dust is initially returned to the kiln, but eventually its alkali or sulfate level becomes so great that it is not suitable for manufacturing cement and must be discarded. This presents a disposal problem. Those sulfur compounds that do not combine with alkalies or lime leave with the flue gases. If sulfur alkalies or other particulate matter are sufficiently high in quantity, the flue gas stream may become environmentally unacceptable and require treatment to meet emission standards.
In short, the rotary kiln process commonly employed for manufacturing portland cement requires a large capital investment, and is not a thermally efficient apparatus, Furthermore, the kiln must remain heated, once it is fired, and remain operational unless shut down in a predetermined manner since the thermal shocks encountered upon cooling will damage its refractory and shell which is quite expensive.
In related U.S. Pat. No. 4,213,791 there is described a method of producing portland and other hydraulic cements in an electric furnace, thereby eliminating the need for a rotary kiln. An important feature of the process described in U.S. Pat. No. 4,213,791 is the ability to utilize a wide variety of feed materials including naturally occurring calcareous and argillaceous materials, as well as by-products from industrial processes, irrespective of whether those materials are in a molten or a pulverulent or non-pulverulent solid state. Plants utilizing the concept of the aforesaid patent are substantially less costly than a conventional rotary kiln and can be considerably sized down, permitting plants to be constructed in locations closer to the point of use of the cements and, thus, reducing shipping costs. Additionally, plants using the process of the type described in U.S. Pat. No. 4,213,791 can be interrupted to the extent that it is completely shut down without damaging equipment used in the process. In addition, it affords the opportunity to select the most energy-efficient drying, preheating, and calcining apparatus available that best suits the raw cement feed materials.
It has been determined that in practicing the process described in the aforesaid U.S. Pat. No. 4,213,791, a substantial advantage is obtained if the furnace is lined with a lining material having substantially or the same composition of the desired cement. Specifically, in a preferred practicing of the process as generally defined in the 4,213,791 patent, a melt is maintained within a cavity within the furnace which is lined with the desired cement, with the melt having substantially the same chemical composition as the desired cement. Appropriate feed materials are introduced into the cavity with the feed material being proportioned to, upon combining chemically, produce the desired cement. The melt is heated within the cavity sufficiently to enable the materials to liquefy and chemically combine within the melt, with the heating being effected by electrical energy. Melt is periodically or continuously withdrawn from the cavity and cooled. Cooling of the withdrawn melt is controlled to solidify it into a substance that has the chemical constituency and properties of the desired portland or hydraulic cement. This improvement is specifically described in copending application Ser. No. 06/285,452 as is a furnace having a shell and the filling of the shell with a filling having substantially the same chemical composition as the cement, a skull in the filling, and means for producing sufficient heat within the skull to maintain a melt having the chemical composition of the desired cement.
Additional improvements and preferred embodiments in the process described in U.S. Pat. No. 4,213,791 have now been discovered and developed. These improvements include
(1) the nature in which the melt is withdrawn from the furnace and cooled to consistently provide a hydraulic cement having the desired constituency; PA1 (2) the manner in which the raw material to produce the cement is fed into the melt as maintained within the furnace; PA1 (3) the apparatus and manner in which the melt is electrically heated, including the particular manner in which the plasma arc torches used in the preferred embodiment of the process are mounted within, and on the furnace; PA1 (4) the manner in which the lining within the furnace is provided and maintained, including in conjunction with the volume and ratio of the raw materials being fed to the furnace; PA1 (5) the manner in which the ratio of feed materials is fed to the furnace to maintain the proper proportions of calcium and silica within the melt which ultimately forms the cement having the desired composition; and PA1 (6) the manner in which a gas for use in the particular electric torches is produced within the system and ultimately fed to the torches so as to avoid noxious gases being passed to the atmosphere.
These and other advantages will be apparent from the following detailed description with emphasis being directed to the accompanying figures of the drawing which form a part of the present specification and wherein like numerals and letters refer to like parts wherever they occur.